This application claims the priority of German Patent Application No. 198 32 386.7, filed Jul. 18, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a reforming reactor. The reactor contains (1) a reforming stage for converting a starting material mixture into a reformate product; (2) a CO shift stage to reduce the CO concentration in the reformate product; and (3) a catalytic burner unit. The catalytic burner unit is in thermal contact with the reformer stage by a heating area and is in thermal contact with the CO shift stage by a cooling area. In the cooling area, the burner unit has less catalyst activity than in the heating area. Preferably, the burner catalyst activity in the cooling area is equal to zero, because only the heating area is lined with a combustion catalyst material, while the cooling area remains catalyst-free. Such reactors are particularly suitable for steam reforming of methanol in a fuel cell vehicle in order to generate the hydrogen necessary for operating the fuel cells from liquid methanol carried on board.
Patent EP 0 529 329 B1 discloses a reforming reactor of this type in the plate stack design, which serves in particular for endothermal reforming of municipal gas. The reactor conducts the resulting hydrogen-rich reformate gas to a stationary phosphoric acid fuel cell system, as used in hotels, apartments, and hospitals. A heat exchange stage is provided between the reformer stage and the CO shift stage. In this heat exchange stage, the reformate gas leaving the reformer stage, before entering the CO shift stage, heats the municipal gas fed to the reformer stage while cooling. In the reformer stage, a reformer layer lined with a reforming catalytic material is in thermal contact with a heating layer of the burner unit, which is lined with combustion catalytic material. Air that has passed through a first catalyst-free cooling layer of the CO shift stage is fed to this heating layer. The fuel used for the catalytic burner unit is the anode output gas of the fuel cell system. The anode gas first passes through a second cooling layer of the CO shift stage and then is fed into a fuel supply layer of the reformer stage. The fuel supply layer is connected with the heating layer via a perforated plate which distributes the fuel evenly as it passes through its openings to the heating layer. The fuel is burned there with air that flows through the heating layer co-currently with the municipal gas fed through the reformer layer. In the CO shift stage, a CO conversion layer lined with suitable CO shift catalytic material, in which the exothermic CO shift reaction takes place, is in thermal contact with the first and second cooling layers adjacent on both sides. The air and the fuel cell anode output gas are separately fed through the cooling layers in a cross-current to the reformate gas flowing through the CO conversion layer.
In another reforming reactor, as disclosed in laid-open patent EP 0 199 878 A2, a tubular catalytic burner unit is provided that is surrounded annularly by a reformer stage, which transitions at an axial end area to a CO shift stage surrounding the reformer stage, likewise annularly. At a U-shaped inlet area, the CO shift stage adjoins the reformer stage separated therefrom by a gas-permeable partition. At its radially internal side, the U-shaped inlet area of the CO shift stage abuts an interior inlet area of the burner unit, which is upstream of its catalytically lined heating area and separated therefrom by a gas-permeable partition.
It is an object of the present invention to provide a reforming reactor that has a compact design, produces sufficiently CO-lean reformate product, and is also especially suitable for mobile applications, for example, in a fuel cell vehicle.
The present invention achieves this object by providing a reforming reactor in which the combustion gas, namely the combustible gas mixture for the catalytic burner unit, is fed counter-current-wise to (1) the reformate product flowing through the CO shift stage, thereby being in thermal contact over at least half the flow path length of the CO shift stage and/or over at least a part thereof on the outlet side (i.e., corresponding to a cooling area); and (2) the starting material mixture flowing through the reformer stage (i.e., corresponding to a heating area of the catalytic burner unit). To carry out its heating function, the heating area is lined with a combustion catalytic material which provides sufficient burner catalyst activity. On the other hand, the cooling area is designed with lower burner catalyst activity, and can be made for example without any catalyst lining, so that no heat of combustion is generated in this area.
This combustion gas feed according to the counter-current principle makes comparatively effective cooling of the CO shift stage possible, so that the CO shift stage and the reactor as a whole can be compact. This contributes to the combustion gas being fed through cooling area of the catalytic burner unit as a homogeneous cooling flow, namely the fuel and the oxygen-containing gas are already mixed. The counterflow of the combustion gas in the heating area in thermal contact with the reformer stage is advantageous for cold starting performance, as the intake side of the reformer stage is maximally heated when starting so that it rapidly reaches the operating temperature necessary for the reforming reaction. A high operating temperature in the reformer stage is desirable so that operation at high load is possible and a high reforming efficiency can be achieved with a compact design. The reformer stage output side, which is in thermal contact with the input side of the heating area due to the counter-current principle, is heated to a correspondingly lower degree. Thus, a desired temperature drop is introduced into the transition area to the CO shift stage, which shifts the equilibrium in the reformate product in the direction of a low CO percentage. Because of this, the CO concentration in the reformate product is brought to a sufficiently low value by the time it reaches the outlet of the CO shift stage, so that the CO shift reaction simultaneously increases the hydrogen yield and hence the total efficiency of the reactor. Because the CO concentration at the outlet of the CO shift stage is already relatively low, if a gas purification stage such as a CO oxidation stage is added downstream for further CO reduction, it does not have to have a particularly high performance and can therefore also be compact.
Another reforming reactor according to the present invention makes a particularly compact system design possible because the reformer stage and the CO shift stage are integrated into a common reaction chamber. The common reaction chamber is in thermal contact by an upstream part with the heating area of the catalytic burner unit and is in thermal contact by a downstream part with the cooling area of the catalytic burner unit. The upstream heated reaction chamber part forms the reformer stage and the downstream cooled reaction chamber part forms the CO shift stage. It must be understood that with this integral construction, the reforming function of the reformer stage makes a smooth transition to the CO reduction function of the CO shift stage.
In an embodiment of a reforming reactor according to the present invention, the cooling area and the heating area of the catalytic burner unit are integrated in a common combustion chamber. Integration of the reformer stage and CO shift stage in a common reaction chamber makes a particularly compact reactor design possible.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.